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Project Gemini was America's second manned space program. Its primary goal was to develop the techniques and equipment needed for the later moon landing, during the Apollo program. The capsule used for Project Gemini could hold two astronauts, stay in orbit for up to two weeks, conduct spaceflights, and dock with other spacecraft. Project Gemini eventually included eleven manned missions, which tested things such as endurance, docking, and maneuvering in outer space.
The Project Gemini spacecraft were launched on the Titan II rocket, a converted intercontinental ballistic missile. The Titan II used hypergolic fuels, hydrazine and nitrogen tetroxide, which ignite upon contact. The Project Gemini capsule itself was much smaller than the Apollo capsule, and could hold only two astronauts in very cramped conditions. The size of the spacecraft was limited by the mass that could be launched into orbit, as well as the need to safely re-enter inside the capsule.
The Project Gemini capsule was the first to include detachable modules; it contained a service module for fuel, power, and life support, a re-entry module to slow down the capsule and return to Earth, and a command module for the astronauts. The Gemini capsule could use rockets to control its movement around the Earth, and often maneuvered into a pre-planned orbit to rendezvous with another vehicle. Originally, batteries were used for power, but hydrogen-oxygen fuel cells were added on in later missions, the first fuel cells to be flown on a manned spacecraft.
Later Project Gemini missions demonstrated the protocols for docking in orbit, and used a docked Agena rocket to boost into a higher orbit. Extra-vehicular activity (EVA), commonly known as “space walking,” was first used on Project Gemini to work on the outside of the spacecraft. Project Gemini astronauts eventually stayed for up to two weeks in space, verifying that the human body and the spacecraft's equipment could hold up during the long voyage to the Moon. After the Apollo project was completed, a larger version of the Gemini spacecraft was proposed to ferry supplies and people to space, but the plan was later scrapped.